In a hydrocarbon stream, saturated and unsaturated organic molecules, oxygen, peroxides, and metal compounds are found, albeit the latter three in trace quantities. Of these materials, peroxides can be the most unstable, decomposing at temperatures from below room temperature and above depending on the molecular structure of the peroxide (G. Scott, "Atmospheric Oxidation and Antioxidants", published by Elsevier Publishing Co., NY 1965).
Decomposition of peroxides will lead to formation of free radicals, which then can start a chain reaction resulting in polymerization of unsaturated organic molecules. Antioxidants can terminate free radicals that are already formed.
Metal compounds and, in particular, transition metal compounds such as copper can initiate free radical formation in three ways. First, they can lower the energy of activation required to decompose peroxides, thus leading to a more favorable path for free radical formation. Second, metal species can complex oxygen and catalyze the formation of peroxides. Last, metal compounds can react directly with organic molecules to yield free radicals.
The first row transition metal species manganese, iron, cobalt, nickel, and copper are found in trace quantities (0.01 to 100 ppm) in crude oils, in hydrocarbon streams that are being refined, and in refined products. C. J. Pedersen (Inc. Eng. Chem., 41, 924-928, 1949) showed that these transition metal species reduce the induction time for gasoline, an indication of free radical initiation. Copper compounds are more likely to initiate free radicals than the other first row transition elements under these conditions.
To counteract the free radical initiating tendencies of the transition metal species and, in particular, copper, so called metal deactivators are added to fluids. These materials are organic chelators that tie up the orbitals on the metal rendering the metal inactive. When metal species are deactivated, fewer free radicals are initiated and smaller amounts of antioxidants are needed to inhibit polymerization.
Not all chelators will function as metal deactivators. In fact, some chelators will act as metal activators. Pedersen showed that while copper is deactivated by many chelators, other transition metals are only deactivated by selected chelators. Moreover, chelators or compounds that may be effective in deactivating a particular metal species, for example, copper, may or may not function effectively to deactivate other metals.
Iron has proven to be a particularly troublesome metal species when contained in liquid hydrocarbons. Liquid hydrocarbon mediums, such as crude oils, crude fractions, such as naphtha, gasoline, kerosene, jet fuel, fuel oil, gas oil and vacuum residuals, often contain metal contaminants, such as iron, that upon processing of the medium, can catalyze undesirable decomposition of the medium or accumulate in the process residue. Accumulation of iron contaminants, like others, is undesirable in the product remaining after refinery, purification, or other processes and, accordingly, diminishes the value of such products.
Iron contamination problems are experienced in conjuction with other liquid hydrocarbons, including aromatic hydrocarbons (i.e., benzene, toluene, xylene), chlorinated hydrocarbons (such as ethylene dichloride), and olefinic and naphthenic process streams. All of the above petroleum feedstock and fractions and petrochemicals are referred to herein as "liquid hydrocarbonaceous mediums".
Iron in such liquid hydrocarbonaceous mediums may occur in a variety of forms. For example, it may be present as a naphthenate, porphyrin, or sulfide. In any case, it is troublesome. For example, residuals from iron-containing crudes are used, inter alia, to form graphite electrodes for industry. The value and useful life of these electrodes is diminished proportionately with the level of undesirable iron contamination.
Additionally, in many processes, iron-containing catalysts are used which may carry over with the product during purification. Iron catalyst contaminated product leads to deleterious effects. This is especially true when FeC1.sub.3 used to catalyze ethylenedichloride (EDC) production "carries over" or persists with the product during purification procedures. Iron or ferric chloride at high concentrations (50-5,000 ppm) causes EDC reboiler fouling to accelerate. Iron contamination causes feedstock degradation forming excessive organic tar and other residues which deposit on heat transfer surfaces, i.e., reboilers. The fouling may reduce plant production capacity and requires extra energy (steam) to run the reboiler properly.